The invention relates to a membrane electrode assembly (MEA) for a fuel cell having a planar polymer membrane, which, in a tangentially inner area, is coated with electrode structure on both sides and, in a tangentially outer area projecting on at least one side beyond the electrode structure coating, is connected to a sealing member.
The invention further relates to a fuel cell stack having a plurality of membrane electrode assemblies arranged between separator plates (bipolar or monopolar separators). Each MEA has a planar polymer membrane, which in a tangentially inner area is coated with electrode structure on both sides and which has a tangentially outer area projecting beyond the electrode structure.
Various types of fuel cells are known in the art. Specifically, in so-called polymer electrolyte membrane fuel cells (PEMFCs), a proton conducting membrane is provided, which is contacted by electrodes on both sides. The electrodes conventionally include a catalytically active layer that is formed of, for example, platinum-coated carbon, which is in direct contact with the membrane, and porous electron conducting structures to transport the reaction gases to the catalytically active layer. These latter structures are usually referred to as gas diffusion structures. They may be formed of, for example, porous carbon paper, carbon fabric or carbon nonwoven material.
To operate the fuel cell, hydrogen gas or a hydrogen-containing gas is delivered to the electrode acting as the anode. The precise composition of the gas depends on the special character of the rest of the fuel cell. At the same time, oxygen gas or an oxygen-containing gas is delivered to the second electrode acting as the cathode. The aforementioned gases are hereinafter referred to as “reaction gases.”
The hydrogen is catalytically oxidized at the anode:H2→2H++2e−.
The electrons released as a result are discharged to the consumer through the electrode, and the protons thus created migrate through the electrolyte to the cathode side where they are converted with oxygen to give water. The required electrons are supplied through the electrode:½O2+2H++2e−→H2O.
In the PEMFC, for example, the charge transfer through the electrolyte occurs through migration of H3O+ ions and/or proton hopping processes. To achieve this, most of the polymer membranes employed must be doped with a doping agent. A frequently used doping agent is, for example, phosphoric acid (H3PO4). Other membranes become adequately ion conductive by absorbing water.
However, doping causes polymer membranes to swell and lose their stability. This makes further handling of the membranes extremely difficult.
For example, mounting a sealing member in the marginal zone of the polymer membrane, which is particularly advantageous for the construction of a fuel cell stack, becomes difficult. The Japanese publication JP 03331873 A1, which discloses a generic MEA, describes a way to circumvent this problem. In the MEA disclosed in that document, the outer area of the polymer membrane lacks electrode structure on one side only. In other words, the electrode structure on the other side of the polymer membrane extends to the margin of the membrane covering also the marginal zone. This produces sufficient stability of the MEA, so that its bare areas can be firmly bonded to a sealing member. The unstable polymer membrane is thus stabilized by the electrode structure, which extends outwardly far beyond the actual electrochemically active inner area of the MEA. This measure has several drawbacks. On the one hand, the enlargement of the electrode area is costly because the electrode material, e.g., platinum coated carbon, is expensive. Secondly, the otherwise desirable goal to use the thinnest possible electrode layer is strictly limited by the increased stability requirements for the electrode layer. Finally, this constellation is also unfavorable electrochemically because the active area of the MEA is not precisely defined. Rather, it extends over a substantially greater area on one side of the polymer membrane than on the other, which can cause problems with ion transport and crossflows.
As an alternative to mounting a sealing member to the MEA itself it is also known to arrange sealing material on the separator plates of a fuel cell stack and to dispose highly flexible, doped MEAs whose polymer membranes are uncoated in the marginal zone on both sides between the separator plates in such a way that the protruding membrane areas interact with the sealing material. A generic fuel cell stack of this type is disclosed in the German publication DE 101 21 176. German publication DE 102 51 439 A1 also discloses a corresponding fuel cell stack. A drawback, however, is the complexity of the stack construction because of the need for additional sealing material and the difficulty of handling the mechanically highly sensitive MEAs.